The present invention relates to fueling tanker truck vapor vent valve assemblies and more particularly to those assemblies with automatic liquid shut-off in response to tank truck roll over.
Fueling tanker trucks are typically constructed as an elongated, horizontal tank having a number of internally divided compartments with various unequal volume capacities to store and transport gasoline, fuel oil, aircraft fuels, or other volatile liquids and fuels. Gasoline tankers are typically bottom loaded through load only or load/unload couplers located below the tanker undercarriage. Vapor rails, channels, or trunks extending along the inside top of the tank include pressure valves tending to generally equalize the vapor pressure at the top of each compartment.
Normally one or more tanker compartments are provided with a large manhole or hatch at the top of the tank. The manhole is covered by a hinged hatch cover which must be sealed liquid tight when closed. Gaskets and other devices are provided for this purpose. One or more of the hatch covers or the upper tank wall itself mounts a vapor pressure relief valve or vent valve operable to allow the escape of vapors from the top of a tank compartment to the atmosphere when excessive pressure builds above a predetermined PSI, such as 3.5 psi or other design parameter. The vent valve also enables the intake of air to relieve predetermined excessive vacuum or negative PSI at the tank top, such as minus 6 ounces PSI. These positive and negative psi conditions normally appear during loading, unloading, a rise in tank temperature or a decline in tank temperature. Unsealed weather caps usually cover the vent valves to prevent rain, snow, or other environmental access to the top of the vent valve. See, for example, "Bottom Loading and Vapor Recovery for MC-306 Tank Motor Vehicles", U.S. Transportation Department, API Recommended Practice 1004, Seventh Edition, November 1988, published by the American Petroleum Institute, incorporated herein by reference.
Because tankers can roll over under certain driving conditions, the vent valves can be a source of stored liquid spills creating a very dangerous situation or damage to the environment. Spills through the vent valve have occurred because of the positive liquid pressure head exerted against the vent valve when the tanker rolls to a degree that places the hatch cover below the liquid level.
Attempts have been made to avoid vent valve spills by relying only on the weight of an axial member to seal against liquid pressure. See Betts Industries Inc., Part No. 6238AL, PPV Pressure Vacuum Vent For MC 306 Tank Vehicles.